1. Field of the Invention
The present invention relates to carrier particles used in the two-component dry-process developers employed in electrophotographic processes. More particularly, this invention relates to a silicone resin composition of specified composition useful for coating these carrier particles.
2. Background Information
In the electrophotographic process, a two-component dry-process developer is brought into contact with an electrostatic latent image on a photosensitive material in order to transfer and adhere the toner to the electrostatic latent image. This toner is subsequently transferred to the receiving sheet and is then fixed at elevated temperature. The two-component dry-process developer used in the electrophotographic process consists of a toner and carrier. The toner consists of thermoplastic resin and pigment, while the carrier consists of iron powder, glass powder, and similar materials. The toner is carried on the surface of the carrier through triboelectrification. In order to avoid the formation of a toner film on the carrier's surface (spanting), the surface of the carrier is coated with a cured film of a silicone resin or a similar toner releasing material.
Numerous silicone resin compositions are already known for the purpose of coating the surface of the carrier in two-component dry-process developers. Examples of useful silicone resins include but are not limited to compositions comprising a silanol-containing organopolysiloxane (Japanese Patent Application Laid-Open [Kokai or Unexamined] Number 56-106968 [106,968/1981]), an organotin compound and an organopolysiloxane comprising difunctional siloxane units (D unit) and trifunctional siloxane units (T unit) (Japanese Patent Application Laid-Open Number 61-284775 [284,775/1986]), and a silicone resin composition composed of a methylphenylsiloxane resin exhibiting a molar ratio of Si-bonded organic groups to silicon atoms no larger than 1.5 (Japanese Patent Application Laid-Open Number 2-160259 [160,259/1990]).
Each of these prior art silicone resin compositions requires heating to 200.degree. to 250.degree. C. after coating on the carrier surface in order to bring about curing of the coated film. This requirement also places limitations on the range of useable carriers. Moreover, due to the high electrical resistance of the cured films derived from silicone resins, the use of carriers coated with such resins in two-component dry-process developers has been associated with problems such as edge development, low image density, and the like. As a consequence, another problem confronting the prior art has been the necessity to adjust the electrical resistance of the silicone resin and find a silicone resin-coated carrier that affords a toner charge in the range of 10 to 30 microcoul/g.
Prior methods for carrier preparation have also involved dilution of the silicone resin composition with large quantities of organic solvent. This organic solvent must be removed after the silicone resin composition has been coated on the carrier surface, which results in degradation of the working environment.
The present invention was arrived at as the result of extensive investigations directed at solving the foregoing problems associated with prior art carrier coating materials.
One object of the present invention is the introduction of a silicone resin composition suitable for carrier coating and a process for carrier production that are both free of the problems associated with prior art resins and processes.
A second objective of this invention is to provide a carrier-coating silicone resin composition that, after coating on the carrier surface, cures rapidly at temperatures no greater than 150.degree. C. to yield a cured film having an optimal electrical resistance value.
Another objective of the this invention is to provide a carrier production method that does not require use of organic solvents.